This invention relates to the photoelectrophoretic imaging process and more particularly to an improved process wherein the imaging suspension is uniformly charged.
A detailed description of the photoelectrophoretic imaging process and materials and apparatus therefore appears in U.S. Pat. Nos. 3,383,993; 3,384,488; 3,384,565 and 3,384,566. The disclosures of the aforementioned patents are hereby incorporated by reference. Briefly, the photoelectrophoretic imaging process, as described in the aforementioned incorporated patents, is a method wherein a liquid suspension of electrically photosensitive particles is placed between a pair of electrodes. The particles acquire a charge when an electrical field is placed between the electrodes which charge is modified by exposure of the particles to light thus causing a light controlled deposition of the particles on one boundary of the suspension or the other. Particle movement is caused by the force exerted on the charged particles by the electric field. The light absorbed by a particle enables it to undergo a change in polarity which then determines its position in the field. One of the electrodes in the process is termed a conductive electrode which is generally a transparent conductive material and is the electrode upon which the pigments desirably rest at the time they are exposed to appropriate electromagnetic radiation. While not subscribing to any particular theory, the aforementioned patents propose that the pigments when exposed to actinic electromagnetic radiation while resting upon the conductive electrode, acquire a charge from the electrode. Upon acquisition of such charge the particle moves toward the opposite electrode. The opposite electrode is generally covered with an electrically insulating material such that when a pigment particle contacts the electrode under influence of the field it will not give up any charge and will remain against the blocking electrode. Upon separation of the electrodes there is generally provided an optically positive image on one of the electrodes and a negative image residing on the other electrode either monochromatic or polychromatic depending upon the optical input and the colors of the pigments in the imaging suspension.
As previously mentioned, the pigments in the imaging suspension have an initial charge and also can acquire an additional electrical charge upon being subjected to an electrical field between the electrodes. One of the problems encountered in the above-mentioned process relates to the polarity of the charge acquired by the pigments of any one color. For example, while about half of certain magenta pigment particles may exhibit a negative charge in the field between the electrodes while in the dark, the other half of the pigment particles will acquire the opposite charge and thus migrate immediately, in the dark, to the blocking electrode. Thus because only some of the pigment resides on the conductive electrode, the density of the resultant image on the conductive electrode is reduced by the amount of pigment deposited on the blocking electrode. Another disadvantage of this phenomenon is the unwanted deposition on the blocking electrode of such pigments in background areas thus degrading image quality of both images produced by the process.
The problem of non-uniform charging of the pigments in the imaging suspension of the photoelectrophoretic imaging method is well known and, in fact, has been employed advantageously in the prior art. For example, a photoelectrophoretic imaging system taking advantage of the diversity of the dark charge of the pigments is disclosed in U.S. Pat. No. 3,535,221, to Tulagin. In accordance with the system disclosed therein the image sense, optical positive or optical negative, is controlled such that one may produce a positive image or a negative image on either of the blocking electrode or the conductive electrode. The method of selectively producing positive or negative copies on either electrode is achieved by providing an imaging suspension with pigment particles having a sensitivity to a first range of wavelengths and providing on the blocking layer surface a photosensitive material sensitive to a second range of wavelengths. In accordance with the disclosure of that patent, an optically positive image is formed on the blocking electrode by exposing the suspension and blocking layer to light in wavelength in which only the coating on the blocking layer is sensitive. If one desires to produce an optically positive image on the conductive electrode one exposes the imaging suspension to electromagnetic radiation to which the particles of the imaging suspension are sensitive but to which the material on the blocking layer is insensitive. The positive image is formed on the blocking layer because of the diversity of charge acquired by the particles of the imaging suspension in the dark. Some of the imaging pigments are attracted to the blocking electrode to form a coating of imaging pigment particles. When the material on the blocking layer is exposed to actinic radiation the pigment particles of the suspension are repelled from the blocking layer in the exposed areas. According to the patent, the coating on the blocking layer reflects back any pigment attracted to it when the coating on the blocking layer is struck with light to which its coating is sensitive. Thus, the light exposed areas will contain no pigment while there will reside on the blocking electrode in non-light struck areas a coating of pigment which has taken an opposite charge to those coating the conductive electrode. When the imaging suspension is exposed with light to which it is sensitive, but to which the coating on the blocking layer is insensitive, the imaging pigments coating the conductive layer are caused to migrate to the blocking electrode in the exposed areas. There is thus produced a positive image configuration of the imaging particles on the conductive electrode.
An even more severe problem exists with polychromatic images produced by the aforementioned process because the loss of varying amounts of pigments of the different colors in this suspension destroys the color balance intended to produce the desired final result.
There has recently been discovered a method whereby all of the pigments of the imaging suspension are uniformly charged thus overcoming the above-mentioned deficiencies. This method involves the coating of the blocking layer with a material which, while in the dark and under the influence of an electrical field, injects charges into the pigments of the imaging suspension. Such materials are termed "dark charge injecting materials" as more fully described below.
Dark charge injecting materials have been coated on the blocking layer by various means such as vacuum depositing an evaporated agent or by painting a liquid dispersion of the agent onto the blocking layer and allowing the liquid to evaporate leaving the agent on the layer. It is also possible to place the agent on the blocking layer by electrophoretic means. However, each method requires cumbersome equipment or extra process steps for the purpose of placing the dark charge injecting agent on the blocking electrode.
In addition, the dark charge injecting material on the blocking electrode has a tendency to transfer from the blocking electrode along with the image produced during the imaging process thus depleting the supply of the dark charge injecting material on the blocking layer and causing unwanted background material on the transferred image.
In an attempt to improve the photoelectrophoretic imaging process which employs dark charge injecting materials on the blocking layer there has been suggested the use of binder materials for the dark charge injecting materials so that they may more permanently reside on the blocking layer. However, such material have included only viscid organic liquids which have been found to be inconvenient for practical use in a commercial machine. Such viscid liquids carry the dark charge injecting material but are slowly eroded with long periods of use in contact with the hydrocarbon liquids normally employed in the imaging suspension of the imaging process.